Field of Invention
The present application relates to the removal of contaminants from stormwater.
Prior Art
The 1972 Clean Water Act (CWA), amended in 1987 to include stormwater discharges, has been the basis of increasing regulatory controls of both point and nonpoint source pollution. The CWA requires stormwater discharges to be regulated through Pollutant Discharge Elimination System (NPDES) permits. As part of these stormwater permits, facilities are often required to implement pollution prevention plans, which identify potential sources of pollution and describe and ensure the implementation of practices that reduce the pollutants in stormwater discharges. These practices are referred to as best management practices (BMPs). Best management practices that provide treatment of stormwater runoff include those that utilize filtration or infiltration. Relatively low loading rate practices such as sand filters and infiltration systems have proven effective in removing pollutants from storm water runoff. For relatively higher loading rate applications, for example at sites where space is constrained, there is a large number of proprietary “ultra-urban” canister or cartridge based filter systems available which comprise of individual canisters or cartridges. Infiltration systems (basins and trenches) are practices in which stormwater runoff is stored in the basin or in a trench, in voids between the stones or in the high void storage structures, and slowly infiltrates into the soil matrix below.
One of the most significant drawbacks to filtration and infiltration systems is premature clogging of the surface due to buildup of solids. In industrial and drinking water treatment, filters are backwashed periodically to restore hydraulic flow, or other mechanical methods may be used (U.S. Pat. No. 7,163,630). This is not possible in typical stormwater filtration devices which operate passively (without power) under gravity filtration. Many stormwater devices are therefore part of systems that incorporate some combination of filter media, hydrodynamic sediment removal, oil and grease removal, or screening to remove floatables and particles that could prematurely blind the filter media. A typical sand filter system includes a pretreatment or sedimentation chamber that prolongs filter media life by removing floatables and heavier suspended solids. Infiltration systems are normally combined with pretreatment such as grass strips or swales, or sediment basins to prevent premature clogging. Dual layer filters in which a coarse grain size media layer is on top of a finer grain size layer is another technique used to prolong the hydraulic life of filters (U.S. Pat. Nos. 7,045,067; 5,281,332; 3,876,546). Ultra-urban filter devices incorporate vertical flows or use relatively coarser media or media in pellet form to prevent premature clogging of the media (U.S. Pat. Nos. 7,419,591; 6,649,048; 5,707,527). In general, the trade off is between using small grain size filter media which improves treatment but results in quicker clogging, and using coarser grain size media which provides poorer treatment but prolongs hydraulic capacity.
In industrial applications, high contact area filter elements are used together with backwashing to prolong the service life of pressurized filtration systems. These elements, called “filter candles”, are typically tubular or cylindrical and consist of slotted or perforated material, such as wire mesh or wedge wire, of various slot or opening sizes to suit the application (U.S. Pat. No. 5,474,586). Increasing the filtration contact area using a removable device has also been proposed for molten polymer filtration (U.S. Pat. No. 3,044,628). Filter candles have recently been proposed in wastewater and stormwater treatment applications (U.S. Pat. Nos. 3,875,055, 4,235,724; 5,492,635; 6,337,025; 6,533,941). Cartridge type filter devices used to treat environmental flows, including stormwater runoff, can also utilize screens both to filter particulates and as a means of directing flows in and out of the individual cartridges (U.S. Pat. Nos. 6,649,048; 5,024,771).
It is established that screen filtration elements are constructed of strong, durable materials such as metals (e.g., stainless steel, aluminum) or thermoplastic polymers (e.g., polypropylene, nylon). These materials, herein referred to as “conventional materials”, do not undergo significant degradation when exposed to an environment (e.g., microorganisms, light, heat, stress, hydrolysis, oxidation) over tens, hundreds, or more years. Degradable materials, such as degradable polymers or polymer composites, are never used for screen material because their degradation results in relatively low strength and durability. The term “degradable material” is used herein to represent a material for which the time span for degradation is substantially shorter than that for conventional materials. Degradation is defined herein as a process of change in the structure of a material resulting in a significant loss of properties (e.g., integrity, weight, structure, mechanical strength, substance) and/or fragmentation into smaller pieces when exposed to an environment (e.g., microorganisms, light, heat, stress, hydrolysis, oxidation). Generally, the time span for degradation for objects made from degradable materials is substantially shorter than the time span required for degradation of objects made from conventional materials having the same dimensions.
Degradable materials can degrade by any number of processes, including, but not limited to, biodegradation, photodegradation, hydrolytic degradation, thermal degradation, oxidative degradation, mechanical degradation, or any combination of these. A biodegradable material, such as a biodegradable polymer or polymer composite, is a material that degrades owing to the action of micro- and/or macroorganisms or enzymes. The rate of biodegradation can vary depending on the nature of the functional group and degree of complexity. Biodegradation processes can occur in a number of ways, including, but not limited to, processes that result in mechanical damage, direct enzymatic effects leading to breakdown of the material structure, and secondary biochemical effects caused by excretion of substances that can directly affect the material or change environmental conditions, such as pH or redox conditions. Microorganisms produce enzymes that catalyze reactions by combining with a specific substrate or combination of substrates. A photodegradable material, including a photodegradable polymer, is a material in which the degradation results from the action of light such as daylight or sunlight. A hydrolytically degradable material, including a hydrolytically degradable polymer, is a material in which the degradation results from hydrolysis. A thermally degradable material, including a thermally degradable polymer, is a material which degrades when heated or when exposed to relatively high temperatures. An oxidatively degradable material, including an oxidatively degradable or oxo-biodegradable polymer, is a material in which the degradation results from oxidation. A mechanically degradable material is one that breaks down relatively easily when force is applied.
It will be understood by those skilled in the art that there are a large and growing number of materials that are degradable materials or can be used as additive, fillers, binders or catalysts to produce degradable materials. Degradable materials can be natural materials, synthetic materials, or a combination of the two, and include, but are not limited to, proteins (e.g., wheat, soy, zein), polysaccharides (e.g., chitin, cellulose, starch, dextran, xanthan, pectin, alginate), and polymers (e.g., degradable polyesters, degradable PP, PGA, PLA, PHA, PHB, PCL, PVOH, EVOH, PBS/PBSA polyesters, PEF, biodegradable PET, copolyesters, polyvinyl alcohol, polyamides, Biomax®, Biopol®, polyurethanes, polyolefins, modified PET, degradable polypropylene), as well as blends of these and other materials. Increasingly, additives are added to conventional polymers and degradable polymers to impart controlled degradation behavior (e.g., catalytic transition metal compounds such as cobalt stearate or manganese stearate).
Degradable materials can undergo degradation when they are buried in soil or other granular media and exposed to the surrounding environment. Degradation can also be induced or the rate of degradation can be increased by manually inducing the degradation. The term “manually inducing” means manipulating the surrounding environment in order to bring about the desired degradation. An example of manually inducing degradation is introducing water into the media to bring about degradation of hydrolytically degradable material. Another example is to provide heat to bring about degradation of thermally degradable material. It will be understood by those skilled in the art that manually induced degradation may be desirable because it allows greater control over the rate of degradation. The rate and manner of degradation of a material is affected by a number of factors, including, but not limited to, temperature, availability of oxygen or lack of it, burial and depth of burial of the product, humidity or wetness, rainfall, size, weight, surface area of product, polymer composition, including polymer type, molecular weight, crystallinity, orientation, surface-to-volume ratio, pH, and environment in which the product rests.
Degradation rates can be measured using a variety of short- or long-term tests including, but not limited to, environmental chamber tests in which the temperature and humidity of the environment can be manipulated, water tests, microbiologically active tests (such as the aerobic and anaerobic tests recommended by ASTM), and composting tests that simulate soil degradation. It will be understood by those skilled in the art that there are a wide variety of United States and International tests available, such as ASTM and ISO tests for biodegradable and compostable materials. The present invention is not necessarily limited to materials that are classified as degradable using one or more of these tests. It is desirable that the degradable material has sufficient balanced degradability characteristics such that it degrades under normal environmental conditions rapidly enough to dissipate in the environment, yet slowly enough that it will not degrade during normal shelf life, storage or shipment time periods, and during installation.
It is an object of this invention to use a specially shaped device constructed wholly or partly from degradable materials, such as degradable polymers, to improve the service life of granular media used for treating contaminated stormwater. The improved service life is achieved by using the device to increase the area available to pass the stormwater flow. It is also an object of this invention to provide some treatment of stormwater runoff by removing pollutants and particles, to increase volume capture of the runoff, and to increase microbial activity in the media.